Nearly all data processing system users are concerned with maintaining back-up data in order to insure continued data processing operations should their data become lost, damaged, or otherwise unavailable.
Many users of data processing systems require continuous availability to stored data during a major disaster that may cause stored data at a single site to become unavailable. For example, banks, insurance companies, and stock market traders take tremendous steps to insure back up data availability in case of a major disaster.
Remote data mirroring is a way of performing data processing operations upon a primary copy of data while continuously updating a secondary copy at a location remote from the primary copy. If the primary copy becomes lost, damaged, or otherwise unavailable, then data processing may continue by accessing the secondary copy at the remote location. Various modes of synchronization can be employed to select a trade-off between the processing delay in updating the secondary copy during normal operation and the processing delay in switching access over to the secondary copy when the primary copy becomes lost, damaged, or otherwise unavailable. See, for example, Yanai et al. U.S. Pat. No. 7,240,238 issued Jul. 3, 2007, incorporated herein by reference.
If it is desired to switch over read-write access from the primary copy to the secondary copy with virtually no disruption of data processing operations, then remote data mirroring is selected to use a synchronous mode of remote data mirroring. The synchronous mode of operation ensures that when a host computer receives an acknowledgement of completion of a transaction of read-write operations upon the primary copy, the transaction of read-write operations will most certainly be performed upon the secondary copy as well. If the host computer finds that the primary copy becomes unavailable, then host computer can switch read-write access over to the secondary copy and resume read-write access (by re-doing the interrupted transaction) without any further recovery operations.
The synchronous mode of remote data mirroring becomes more demanding with increased distance to the remote secondary copy. There is an inherent delay in transmitting an update to the remote secondary copy and receiving back an acknowledgement of receipt. Therefore in the ideal case it is desired to have a dedicated data link to the remote secondary copy, and in the ideal case it is desired for the dedicated data link to have sufficient bandwidth to carry updates from peak loading of read-write operations from the host computer. In practice, a dedicated data link is relatively costly in comparison to alternatives such as the Internet or on-demand access to additional lines in a public telephone network. Therefore various methods have been used for mitigating delay or disruption due to peak loads and intermittent availability of transmission bandwidth to the remote secondary copy. See, for example, Wilson et al. U.S. Pat. No. 7,647,460 issued Jan. 12, 2010, incorporated herein by reference, and Wahl et al. U.S. Pat. No. 7,562,250 issued Jul. 14, 2009, incorporated herein by reference.